In the field of testing of liquid analytes, particularly those using the so-called dried chemistry technology, a quantity of patient liquid is dispensed onto a test slide element having at least one dried reagent layer with which the liquid interacts to produce a detectable signal for sensing the analyte of interest.
It has been required with clinical diagnostic analyzers, such as those manufactured by Eastman Kodak Company under the trademark "EKTACHEM.TM.", which use the dried chemistry assay approach, that serum be separated from the whole blood sample prior to testing of the sample. Techniques for performing the separation are known and typically require centrifugation of a liquid sample to separate the heavier phase, (e.g., that portion carrying the red blood cells), from the lighter phase, (e.g., serum). These separation techniques are expensive in that they require a centrifuge, or other apparatus, to perform the separation. In addition, these techniques are time consuming in that they are usually performed offline and separate from the analyzer prior to testing of the filtered sample.
Alternatively, there are known filtering techniques in which a quantity of patient liquid is added to one end of a filter and separated. The filtered liquid then directly passes into a container for holding the filtered liquid from where the liquid can be aspirated into a conventional pipette tip for metering onto a test element. Alternatively, the separated liquid may be retained at one end of the filter after being passed therethrough.
There are associated problems with each described technique. In the first described method, additional processing steps are required to first collect the filtered liquid and subsequently to aspirate it into a tip for dispensing. In addition, using multiple pipette tips and containers increases the number of disposables.
There are also a number of potential problems associated with the metering of a sample, filtered or otherwise, using conventional point source deposition methods.
In the most common point source method, the pipette tip approach, a predetermined quantity of liquid is dispensed from the orifice of a pipette tip suspended a short distance above the test element. The point source delivers the liquid in the form of a droplet which strikes the test element in a localized area. Biological liquids, such as serum, however, vary in terms of their spreadability (diffusivity) horizontally across a test element surface. For example, a patient sample having a higher concentration of lipids, or lipo-proteins, will not spread as easily as one with a lower concentration due to its higher viscosity. The inability to spread, and further the inability of some liquids to spread homogeneously, may produce inaccuracies in the detection of an analyte of interest.
The accurate dispensing of the liquid onto the surface of a test element is also dependent upon a number of other factors, such as adhesion of the liquid to the tip, flow characteristics of the tip nozzle, the distance of the tip from the test element, the air flow in the vicinity of the tip, etc. Spreading layers typically provided for in the chemistry portion of a test element provide for horizontal as well as vertical diffusivity across the dried reagent layer(s); however, it is possible that the spreading will not be homogenous. In addition, the manufacture of a porous spreading layer to horizontally distribute the sample droplet is expensive.
In the second filtering technique described above, there are also a number of associated problems. Firstly, a predetermined quantity of patient liquid cannot be dispensed directly from the wetted end of a filter without potentially flooding the test element. Typically, it is required for only about 10 .mu.l of liquid to be dispensed to the test surface area of a test element. In addition, liquid collected at the wetted end of a filter is prone to excessive contact with the atmosphere causing some of the collected liquid to evaporate prematurely, and require an excess of biological liquid be used.
Therefore, there is a need to provide a method of directly providing whole blood or other unfiltered patient liquid to an analyzer wherein the whole blood can be separated and then transferred onto the test surface of a test element with a minimum of processing steps.
A further need is to provide a container for storing a patient sample from which a number of patient samples could be transferred to the surface of a test element without the need for additional disposable parts.
An additional need is to control the amount of dispensed volume to the test element from a container having filtration means so that a generally uniform and thin layer is transferred directly without the necessity for horizontally flowing the sample as required with point source metering.